theorem secondFF_from_hessian (hJ : 0 < J) (ρ c : ℝ) (hc : 0 < c)
(v : Fin J → ℝ) (hv : orthToOne J v) :
secondFundamentalFormQF J ρ c v =
-cesHessianQF J ρ c v * Real.sqrt ↑J := by
rw [cesHessianQF_on_perp hJ ρ c hc v hv]
simp only [secondFundamentalFormQF]
rw [curvature_eq_neg_eigenvalue_sqrt hJ ρ c hc]
ringDifferential Geometry of CES Isoquants (Gap #6)